Generally, a multipath constant-current control driver for LEDs can be implemented with: 1. a constant-voltage module together with multiple non-isolated DC/DC constant-current circuits (e.g., BUCK circuits); or, 2. a voltage-adjustable voltage regulating module together with multiple linear regulating constant-current circuits.
As shown in FIG. 1, in the first scheme, the output of the constant-voltage module is inputted to the constant-current circuits, and each of the constant current circuits performs constant-current control independently, which can easily ensure the balancing between output currents. However, there is normally a significant disparity between the voltage of the constant-voltage module and the voltage across an LED load; therefore none of the DC/DC constant-current circuits after the constant-voltage module has high efficiency. In addition, the cost of the multipath constant-current control circuit is high.
As shown in FIG. 2, in the second scheme, MOS transistors or triodes are used to carry out linear regulation and hence multipath constant-current control. The output voltage of the voltage regulating module follows the linear regulating constant-current circuits after it, so that the output voltage of the voltage regulating module remains slightly higher than the highest one of the output voltages of the linear regulating constant-current circuits; as a result, power consumption of each of the linear regulating constant-current circuits remains close to the minimum while precise constant-current control is achieved. This scheme has the advantages including low cost of the circuit and good current balancing between the LED loads. However, as one of the most common ways for an LED to fail, short circuits may cause a significant disparity between the voltages across the LED loads, which leads to high power consumption at the linear regulating devices and hence a large amount of heat generated by the LED driver.